Paper coating compositions are generally fluid aqueous suspensions of one or more pigments, such as kaolin, titanium dioxide, calcium carbonate, or alumina; and one or more pigment binder materials derived from starch, protein, or synthetic polymers. Insolubilizing agents for the pigment binders are used to reduce the water sensitivity of the binders and improve the surface strength of the coated sheet, qualities necessary to allow the use of aqueous inks and allow the paper to withstand the mechanical stresses of printing.
Paper sizing compositions are used to improve the water and oil resistance and printability of the paper to which they are applied. They are applied by either of two methods; (1) They can be added to the pulp or furnish immediately prior to the formation of the sheet. This is commonly called "wet end" or "internal" sizing. Internal sizes can be rosin based, often used in combination with alum, alkyl ketene dimer compositions, or alkenyl succinic anhydride compositions. (2) They can be applied in a separate size press or surface coating operation after the sheet has been formed and dried. The insolubilizing agents of this invention are applicable to sizing compositions which are applied by the second method, in a size press or surface coating operation. The sizing compositions applied in this manner are commonly starches or synthetic polymers. The use of insolubilizing agents for these sizing compositions decreases their water sensitivity and improves the printability of the paper produced. For the purposes of this patent, paper sizing compositions applied in a size press or surface coating operation (method 2) will be referred to as "paper coatings" to distinguish them from internal sizing compositions.
A number of insolubilizing agents are commercially available and have been used to insolubilize binding materials in paper coatings and paper surface coatings. Formaldehyde has been used in the past but is no longer widely used due to highly restrictive environmental laws that limit vapor concentrations of formaldehyde to extremely low levels. It is quite effective as an insolubilizer with protein binders but is not as effective with starch and synthetic polymers at the alkane pH values in paper coatings.
Glyoxal does not have the severe environmental restrictions of formaldehyde and is effective with synthetic polymers and starch as well as with protein. However, the addition of glyoxal tends to undesirably increase the viscosity of the coating compositions, which renders the coatings difficult to apply on modern high speed paper making machinery. The use of glyoxal can also cause discoloration of the coating compositions and the papers to which they are applied.
At one time urea formaldehyde and melamine formaldehyde resins were the predominant insolubilizers in the paper industry, and melaminue formaldehyde resins and particularly the alkylated resins are still widely used. However, the resins contain up to about 1% free formaldehyde and can evolve formaldehyde vapors in the drying and curing operations and thus their use requires strict environmental controls. Also, an acid catalyst may be required to effect the cure. In addition they do not cure rapidly and may require an aging period before a complete cure is effected.
Modified glyoxal resins have come into wide use most recently. They do not release formaldehyde vapors, they do not require a catalyst, and the paper is cured immediately on drying. These resins are highly reactive and tend to crosslink and precipitate in concentrated solutions containing more than about 10 percent by weight resin material. Thus they are difficult to manufacture in a stable concentrated form, and a compromise is necessary between the degree of dilution of the agents and the expected shelf life.
Metal salts can also be used as insolubilizers. The most commonly used is ammonium zirconium carbonate, primarily for protein coatings. These products are quite unstable to heaing, dilution, and pH changes. They also give off an undesirable ammonia odor on curing.
Maleic anhydride copolymer derivatives are used extensively in the manufacture of paper, usually as surface coating agents. To prepare them for use in this application, the maleic anhydride copolymers are either neutralized by the addition of basic materials to form salt solutions, or they are modified by esterification or amide formation prior to their addition to or use as coating compositions. The anhydride moieties are destroyed by these treatments and are no longer available to react with and insolubilize the binders and coatings used in these formulations.
A further requirement for reagents to be used in paper manufacture is that they be nontoxic so that the paper products manufactured can be used for packaging foods without introducing harmful substances.
It would be desirable to have effective insolubilizing agents for paper coatings which do not have adverse toxicity or environmental effects, do not cause undesirable color formation in the paper, do not increase the viscosity of the paper coatings to which they are added, and can be marketed as shelf stable compositions with high levels of reactive functionality.